1. Field of the invention
This invention relates to an image signal processing system for use with a facsimile and the like and, more in particular, to a system for converting an analog image signal obtained by reading an original by means of an image sensor into a binary image signal for further processing such as application to a dot-matrix type recording apparatus or transmission to a remote place.
2. Description of the Prior Art
An image signal processing system for converting an analog image signal into a digital or binary image signal has been well known. For example, in transmitting image data from one place to a remote place, it is preferable to convert an analog image signal obtained by reading an original into a digital signal in the form of the binary numbers "0" and "1" before transmitting to a remote place from various reasons. Such a digital signal may further be converted into compressed codes before transmission as in a digital facsimile communication system. Moreover, a digital image signal represented by the binary numbers is sometimes required at the output end because of the use of a dot-matrix type recording apparatus such as an ink-jet printer and a multi-stylus electrostatic printer.
Although there are numerous advantages in the use of a binary image signal over an analog image signal itself, one of the difficulties encountered in handling the binary image signal is how to deal with an original having tone variations such as a photographic picture. Since the binary image signal only consists of two possible states, high and low, tone usually cannot be shown on a reproduced image and, therefore, a gray area is represented either as "white" or "black". Several techniques have been proposed to represent tone differences with the use of a binary image signal, such as the dither technique; however, none of the prior art techniques is satisfactory.